Microchannel plates (MCP's) are high gain, low noise, solid-state electron multipliers consisting of millions of tiny, alkali doped lead glass channels all fused together into a solid array. FIG. 1 is a photomicrograph illustrating the microchannel structure. These devices are sensitive to a wide range of charged particles and electromagnetic radiation and are fabricated in sizes ranging from 3 to 150 millimeters in diameter.
In operation as shown in FIG. 2, charged particles (ions, electrons) or electromagnetic radiation (UV Photon, Soft X-Rays) impinge on the input side of the array with sufficient energy to generate secondary electrons. The secondary electrons accelerate through the channel toward the output side of the channel, driven by the ever increasing positive electric potential created by current flowing within the resistive layer of the channel structure. Subsequent collisions of the secondary electrons with the channel wall create further secondary electrons in a cascade until the charge exits the channel and is recorded on a readout device. Varying the voltage applied across the array will vary the gain by influencing the number of collisions and the number of secondary electrons generated upon each successive collision with the channel wall.
Typical microchannel plates can produce approximately 10,000 electrons for every single charged particle impinging on the input surface. Microchannel plates can be stacked together in order to obtain improved performance. When two MCP's are stacked, the resultant device has a typical gain of about 10,000,000 (107). Stacking three MCP's together provides a gain of up to about 100,000,000 (108).
Microchannel plates were originally developed for image intensifiers used in night vision scopes. Today, microchannel plates are used in a wide variety of commercial and scientific applications ranging from space exploration (the Hubble Space Telescope contains several instruments employing microchannel plates) to semiconductor processing, to drug discovery, cancer research, and anti-terrorist activities. Microchannel plates are no longer limited to the small formats developed for night vision and are produced in sizes ranging from 3 to 150 mm in diameter or other major dimension. Shown in FIG. 3 are some known product forms of microchannel plates.
The applications in which microchannel plates are used rely on the high sensitivity of the microchannel plate to detect and amplify weak signals, which contain complex information that would not be detected without the use of the MCP. Microchannel plate detectors in medical instruments enable blood analyzers to function. Mass spectrometers with parts per billion analysis capabilities only function when equipped with MCP detectors. Many pharmaceutical and medical breakthroughs of the last 10 years would not have occurred if it were not for microchannel plates. Unlike MCP's used in image intensifier tubes, MCP's for analytical instruments frequently need to be cycled from high vacuum to atmospheric pressure.
In order to operate a microchannel plate it must be mounted in a conductive fixture which makes electrical contact to the electrodes which are formed on each side of the plate. The electrodes are used to apply the high voltage needed to create an electric field within the channels that sustains the secondary electron emission. When microchannel plates were first invented, they had very large pores (i.e., about 50 microns in diameter) and thick channel walls (i.e., about 12 microns thick). They had active channels extending all the way out to the very edge of the MCP as shown in FIG. 4. Making electrical connection to such a structure was accommodated by simply sandwiching the MCP disk between two metal washers.
That structure provided very good support for the MCP and was successfully optimized for high shock and vibration environments. The relatively wide channel walls easily supported the structure with enough surface area to make good electrical contact without causing mechanical damage to the array.
One serious drawback with the known approach is that the sandwiching of the MCP between two metal washers effectively closed off hundreds of channels beneath the metal washers. That results in trapping of gas at atmospheric pressure inside the covered channels. All microchannel plates must operate in a high vacuum environment and therefore, when the MCP was subjected to vacuum, the trapped gas would slowly diffuse from the pores. Such diffusion significantly increased the pump down time for the device. Failure to evacuate these channels thoroughly could lead to ignition of the gas into a plasma when the high voltage was applied to the MCP. The plasma burns the metallized electrodes and may even melt the glass structure, thereby generating noise and rendering the array useless.
Needs in the market place have continuously driven MCP manufacturers to make devices having smaller pores. Smaller pores have thinner channel walls which further complicates the-mounting process because the thinner channel walls may lead to crushing of the channel walls during mounting in an operative device. Crushed channel walls cause noise problems during operation of the microchannel plate.
In an effort to make mounting of small pore (i.e., less than about 25 microns in diameter) microchannel plates more reliable, a solid glass border 12 which completely surrounds a defined active area 14 was used, as shown in FIG. 5. The addition of the solid glass border 12 to the microchannel plate 10 successfully eliminated the problems associated with mounting MCP's which have active channels out to the edge. The addition of the solid glass border did however create a new significant problem.
More specifically, the addition of the solid glass border introduced a severe problem with spontaneous warping and cracking of the microchannel plate. Microchannel plates are manufactured from alkali doped lead silicate glass. The active surfaces of a microchannel plate, within the channels are essentially a fired silica gel. This surface is known to be very hygroscopic, that is, it absorbs water vapor readily from the ambient environment. The composition of the channel walls of a microchannel plate, regardless of glass type or manufacturer, are chemically almost identical to that material used in silica desiccating packs used to absorb water and keep clothing, electronics, and other products dry.
The porous nature of the microchannel plate structure means that the active area can have several hundred times the surface area of the nonporous solid glass rim. When microchannel plates are manufactured they are machined parallel and flat to within 20 microns. When hydration occurs, the active area 14 swells as illustrated in FIG. 6 and begins to expand in the directions illustrated by the arrows. As the active area 14 expands it begins to push against the solid glass border 12 which expands at a much slower rate, based on the difference in the surface area. Continued expansion of the active area 14 causes the microchannel plate 10 to become distorted, i.e., concave on one side and convex on the other. Further expansion of the active area 14 will eventually cause the solid glass border 12 to fail in tension by cracking. The classic hydration failure is characterized by a crack originating at the edge of the MCP 10 and extending toward the center of the MCP. The crack is wider at the perimeter of the solid glass rim 12 than in the center of the active area 14. This behavior can be modeled using hoop stress equations.
Hydration failures may be prevented by keeping the MCP stored in a good vacuum. However, microchannel plates are now used in many applications that require cycling to ambient atmosphere and the continuous vacuum treatment is no longer feasible or cost effective.
In order to resolve this problem it is necessary to build a microchannel plate structure which will tolerate an expansion of the active area and provide a mounting structure which will provide good electrical contact, without damaging the active channels. The desired structure should not trap gas within unused channels.